Spis treści
Security: IPsec
Software installation
Install ipsec-tools and strongswan
Commands to install ipsec:
wget http://launchpadlibrarian.net/234435245/ipsec-tools_0.8.2+20140711-5_amd64.deb sudo apt install ./ipsec-tools_0.8.2+20140711-5_amd64.deb
Command to install strongswan:
sudo apt install strongswan
IPSec configuration via setkey
Modification of configuration files
- setkey.conf_a_
#!/usr/sbin/setkey -f ## konfiguracja dla maszyny A ## flush SAD and SPD flush; spdflush; ## add SAs in SAD add 172.20.253.237 172.20.252.198 esp 0x1000 -E aes-cbc 0xaa223344556677889900aabbccddeeff; add 172.20.252.198 172.20.253.237 esp 0x2000 -E aes-cbc 0xbb223344556677889900aabbccddeeff; ## add SPs in SPD spdadd 172.20.253.237 172.20.252.198 any -P out ipsec esp/transport//require; spdadd 172.20.252.198 172.20.253.237 any -P in ipsec esp/transport//require;
- setkey.conf_b_
#!/usr/sbin/setkey -f ## konfiguracja dla maszyny B ## flush SAD i SPD flush; spdflush; ## add SAs in SAD add 172.20.253.237 172.20.252.198 esp 0x1000 -E aes-cbc 0xaa223344556677889900aabbccddeeff; add 172.20.252.198 172.20.253.237 esp 0x2000 -E aes-cbc 0xbb223344556677889900aabbccddeeff; ## add SPs in SPD spdadd 172.20.253.237 172.20.252.198 any -P in ipsec esp/transport//require; spdadd 172.20.252.198 172.20.253.237 any -P out ipsec esp/transport//require;
Configuration loading
On the machine a:
administrator@ipsec-A:~$ sudo setkey -f setkey.conf_a_ administrator@ipsec-A:~$ sudo setkey -D 172.20.252.198 172.20.253.237 esp mode=transport spi=8192(0x00002000) reqid=0(0x00000000) E: aes-cbc bb223344 55667788 9900aabb ccddeeff seq=0x00000000 replay=0 flags=0x00000000 state=mature created: Jun 14 19:29:57 2025 current: Jun 14 19:30:01 2025 diff: 4(s) hard: 0(s) soft: 0(s) last: hard: 0(s) soft: 0(s) current: 0(bytes) hard: 0(bytes) soft: 0(bytes) allocated: 0 hard: 0 soft: 0 sadb_seq=1 pid=13464 refcnt=0 172.20.253.237 172.20.252.198 esp mode=transport spi=4096(0x00001000) reqid=0(0x00000000) E: aes-cbc aa223344 55667788 9900aabb ccddeeff seq=0x00000000 replay=0 flags=0x00000000 state=mature created: Jun 14 19:29:57 2025 current: Jun 14 19:30:01 2025 diff: 4(s) hard: 0(s) soft: 0(s) last: hard: 0(s) soft: 0(s) current: 0(bytes) hard: 0(bytes) soft: 0(bytes) allocated: 0 hard: 0 soft: 0 sadb_seq=0 pid=13464 refcnt=0 administrator@ipsec-A:~$ sudo setkey -DP 172.20.252.198[any] 172.20.253.237[any] 255 fwd prio def ipsec esp/transport//require created: Jun 14 19:29:57 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=82 seq=1 pid=13511 refcnt=1 172.20.252.198[any] 172.20.253.237[any] 255 in prio def ipsec esp/transport//require created: Jun 14 19:29:57 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=72 seq=2 pid=13511 refcnt=1 172.20.253.237[any] 172.20.252.198[any] 255 out prio def ipsec esp/transport//require created: Jun 14 19:29:57 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=65 seq=3 pid=13511 refcnt=1 (per-socket policy) in(socket) none created: Jun 14 19:18:30 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=59 seq=4 pid=13511 refcnt=1 (per-socket policy) out(socket) none created: Jun 14 19:18:30 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=52 seq=5 pid=13511 refcnt=1 (per-socket policy) in(socket) none created: Jun 14 19:18:30 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=43 seq=6 pid=13511 refcnt=1 (per-socket policy) out(socket) none created: Jun 14 19:18:30 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=36 seq=7 pid=13511 refcnt=1 (per-socket policy) in(socket) none created: Jun 14 19:18:30 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=27 seq=8 pid=13511 refcnt=1 (per-socket policy) out(socket) none created: Jun 14 19:18:30 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=20 seq=9 pid=13511 refcnt=1 (per-socket policy) in(socket) none created: Jun 14 19:18:30 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=11 seq=10 pid=13511 refcnt=1 (per-socket policy) out(socket) none created: Jun 14 19:18:30 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=4 seq=0 pid=13511 refcnt=1 administrator@ipsec-A:~$
On machine b:
administrator@ipsec-B:~$ sudo setkey -f setkey.conf_b_ administrator@ipsec-B:~$ sudo setkey -D 172.20.252.198 172.20.253.237 esp mode=transport spi=8192(0x00002000) reqid=0(0x00000000) E: aes-cbc bb223344 55667788 9900aabb ccddeeff seq=0x00000000 replay=0 flags=0x00000000 state=mature created: Jun 14 19:31:00 2025 current: Jun 14 19:31:15 2025 diff: 15(s) hard: 0(s) soft: 0(s) last: hard: 0(s) soft: 0(s) current: 0(bytes) hard: 0(bytes) soft: 0(bytes) allocated: 0 hard: 0 soft: 0 sadb_seq=1 pid=32009 refcnt=0 172.20.253.237 172.20.252.198 esp mode=transport spi=4096(0x00001000) reqid=0(0x00000000) E: aes-cbc aa223344 55667788 9900aabb ccddeeff seq=0x00000000 replay=0 flags=0x00000000 state=mature created: Jun 14 19:31:00 2025 current: Jun 14 19:31:15 2025 diff: 15(s) hard: 0(s) soft: 0(s) last: hard: 0(s) soft: 0(s) current: 0(bytes) hard: 0(bytes) soft: 0(bytes) allocated: 0 hard: 0 soft: 0 sadb_seq=0 pid=32009 refcnt=0 administrator@ipsec-B:~$ sudo setkey -DP 172.20.252.198[any] 172.20.253.237[any] 255 out prio def ipsec esp/transport//require created: Jun 14 19:31:00 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=17 seq=1 pid=32078 refcnt=1 172.20.253.237[any] 172.20.252.198[any] 255 fwd prio def ipsec esp/transport//require created: Jun 14 19:31:00 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=10 seq=2 pid=32078 refcnt=1 172.20.253.237[any] 172.20.252.198[any] 255 in prio def ipsec esp/transport//require created: Jun 14 19:31:00 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=8 seq=0 pid=32078 refcnt=1 administrator@ipsec-B:~$
Tunnel Action Test
Performing a ping from machine A to B and observing the ESP header
Questions
What traffic is secured by an IPSec tunnel?
IP traffic between machines A (172.20.253.237) and B (172.20.252.198) - any protocol (any) defined in the SPD.
What IPSec protocol is used? Which algorithms?
- Protocol: ESP
- Encryption algorithm: AES-CBC
- Services: Encryption (no authentication in this example)
Role of SAD and SPD:
- SAD: Stores SA parameters (keys, algorithms, SPI).
- SPD: Stores the security policies (which traffic is to be secured and how)
Which commands are processed for outbound/inbound traffic?
- For outgoing packets -.
spdadd … -P out - For incoming packets -
spdadd … -P in
Why do you need two SAs?
SAs are unidirectional - one for A → B, the other for B → A to secure a two-way channel.
Meaning of commands ''setkey -D'' i ''setkey -DP''
-D- shows active security associations (SA)-DP- shows active safety policies (SPD)
What is the purpose of the SPI field in the IPSec header?
The SPI (Security Parameter Index) is a unique security association (SA) identifier that allows the recipient to identify which parameters to use to decrypt and authenticate the packet.
What is the purpose of the Sequence Number field in the IPSec header?
The Sequence Number is used to prevent replay attacks. The number increases monotonically for each packet, allowing you to check that packets have not been replayed.
Modifying IPSec and SA policies
ESP with AES-CBC + HMAC-SHA1
- setkey.conf_a_
#!/usr/sbin/setkey -f ## maszyna A (172.20.253.237) flush; spdflush; ## SA: A->B add 172.20.253.237 172.20.252.198 esp 0x1000 -E aes-cbc 0xaa223344556677889900aabbccddeeff -A hmac-sha1 0x00112233445566778899aabbccddeeff00112233; ## SA: B->A add 172.20.252.198 172.20.253.237 esp 0x2000 -E aes-cbc 0xbb223344556677889900aabbccddeeff -A hmac-sha1 0x11223344556677889900aabbccddeeff00112233; ## SPD: zabezpiecz wszystkie pakiety ESP w trybie transportowym spdadd 172.20.253.237 172.20.252.198 any -P out ipsec esp/transport//require; spdadd 172.20.252.198 172.20.253.237 any -P in ipsec esp/transport//require;
- setkey.conf_b_
#!/usr/sbin/setkey -f ## maszyna B (172.20.252.198) flush; spdflush; ## SA: B->A add 172.20.252.198 172.20.253.237 esp 0x2000 -E aes-cbc 0xbb223344556677889900aabbccddeeff -A hmac-sha1 0x00112233445566778899aabbccddeeff00112233; ## SA: A->B add 172.20.253.237 172.20.252.198 esp 0x1000 -E aes-cbc 0xaa223344556677889900aabbccddeeff -A hmac-sha1 0x00112233445566778899aabbccddeeff00112233; ## SPD: zabezpiecz wszystkie pakiety ESP w trybie transportowym spdadd 172.20.252.198 172.20.253.237 any -P out ipsec esp/transport//require; spdadd 172.20.253.237 172.20.252.198 any -P in ipsec esp/transport//require;
AH with HMAC-SHA1
- setkey.conf_a_
#!/usr/sbin/setkey -f ## maszyna A (172.20.253.237) flush; spdflush; ## SA: A->B add 172.20.253.237 172.20.252.198 ah 0x1000 -A hmac-sha1 0x00112233445566778899aabbccddeeff00112233; ## SA: B->A add 172.20.252.198 172.20.253.237 ah 0x2000 -A hmac-sha1 0x11223344556677889900aabbccddeeff00112233; ## SPD: zabezpiecz wszystkie pakiety AH w trybie transportowym spdadd 172.20.253.237 172.20.252.198 any -P out ipsec ah/transport//require; spdadd 172.20.252.198 172.20.253.237 any -P in ipsec ah/transport//require;
- setkey.conf_b_
#!/usr/sbin/setkey -f ## maszyna B (172.20.252.198) flush; spdflush; ## SA: B->A add 172.20.252.198 172.20.253.237 ah 0x2000 -A hmac-sha1 0x11223344556677889900aabbccddeeff00112233; ## SA: A->B add 172.20.253.237 172.20.252.198 ah 0x1000 -A hmac-sha1 0x00112233445566778899aabbccddeeff00112233; ## SPD: zabezpiecz wszystkie pakiety AH w trybie transportowym spdadd 172.20.252.198 172.20.253.237 any -P out ipsec ah/transport//require; spdadd 172.20.253.237 172.20.252.198 any -P in ipsec ah/transport//require;
ESP (AES-CBC) + AH (HMAC-SHA1)
- setkey.conf_a_
#!/usr/sbin/setkey -f ## maszyna A (172.20.253.237) flush; spdflush; ## SA ESP: A->B add 172.20.253.237 172.20.252.198 esp 0x3000 -E aes-cbc 0xaa223344556677889900aabbccddeeff; ## SA ESP: B->A add 172.20.252.198 172.20.253.237 esp 0x4000 -E aes-cbc 0xbb223344556677889900aabbccddeeff; ## SA AH: A->B add 172.20.253.237 172.20.252.198 ah 0x5000 -A hmac-sha1 0x00112233445566778899aabbccddeeff00112233; ## SA AH: B->A add 172.20.252.198 172.20.253.237 ah 0x6000 -A hmac-sha1 0x00112233445566778899aabbccddeeff00112233; ## SPD: zabezpiecz pakiety ESP i AH w trybie transportowym spdadd 172.20.253.237 172.20.252.198 any -P out ipsec esp/transport//require ah/transport//require; spdadd 172.20.252.198 172.20.253.237 any -P in ipsec esp/transport//require ah/transport//require;
- setkey.conf_b_
#!/usr/sbin/setkey -f ## maszyna B (172.20.252.198) flush; spdflush; ## SA ESP: B->A add 172.20.252.198 172.20.253.237 esp 0x4000 -E aes-cbc 0xbb223344556677889900aabbccddeeff; ## SA ESP: A->B add 172.20.253.237 172.20.252.198 esp 0x3000 -E aes-cbc 0xaa223344556677889900aabbccddeeff; ## SA AH: B->A add 172.20.252.198 172.20.253.237 ah 0x6000 -A hmac-sha1 0x00112233445566778899aabbccddeeff00112233; ## SA AH: A->B add 172.20.253.237 172.20.252.198 ah 0x5000 -A hmac-sha1 0x00112233445566778899aabbccddeeff00112233; ## SPD: zabezpiecz pakiety ESP i AH w trybie transportowym spdadd 172.20.252.198 172.20.253.237 any -P out ipsec esp/transport//require ah/transport//require; spdadd 172.20.253.237 172.20.252.198 any -P in ipsec esp/transport//require ah/transport//require;
Questions
Difference between ESP and ESP+AH configurations
- ESP protects the data (encrypts), but does not protect the entire IP header.
- AH does not encrypt, but protects the integrity of the entire IP packet (with header).
- ESP+AH ensures the confidentiality and integrity of the header and data.
Application of AH
For connections where the integrity of the entire IP header is important (e.g. for auditing systems or where it is not possible to use encryption).
IKE and strongSwan protocol (PSK)
SAD and SPD cleansing
On bush A and B:
sudo setkey -F # usuń wszystkie SA sudo setkey -FP # usuń wszystkie SP
Configurations
Machine A
/etc/ipsec.conf:
config setup
charondebug = "ike 1, knl 1, cfg 1"
conn host-host
keyexchange=ikev2
authby=psk
left=172.20.253.237 # lokalny endpoint
leftsubnet=172.20.253.237/32
right=172.20.252.198 # zdalny endpoint
rightsubnet=172.20.252.198/32
ike=aes256-sha1-modp1024
esp=aes256-sha1
auto=add
/etc/ipsec.secrets:
172.20.253.237 172.20.252.198 : PSK "SuperTajnyPSK"
Machine B
/etc/ipsec.conf:
config setup
charondebug = "ike 1, knl 1, cfg 1"
conn host-host
keyexchange=ikev2
authby=psk
left=172.20.252.198
leftsubnet=172.20.252.198/32
right=172.20.253.237
rightsubnet=172.20.253.237/32
ike=aes256-sha1-modp1024
esp=aes256-sha1
auto=add
/etc/ipsec.secrets:
172.20.252.198 172.20.253.237 : PSK "SuperTajnyPSK"
Starting strongSwan and establishing a tunnel
administrator@ipsec-A:~$ sudo ipsec restart Stopping strongSwan IPsec... Starting strongSwan 5.9.5 IPsec [starter]... administrator@ipsec-A:~$ sudo ipsec up host-host initiating IKE_SA host-host[1] to 172.20.252.198 generating IKE_SA_INIT request 0 [ SA KE No N(NATD_S_IP) N(NATD_D_IP) N(FRAG_SUP) N(HASH_ALG) N(REDIR_SUP) ] sending packet: from 172.20.253.237[500] to 172.20.252.198[500] (1044 bytes) received packet: from 172.20.252.198[500] to 172.20.253.237[500] (344 bytes) parsed IKE_SA_INIT response 0 [ SA KE No N(NATD_S_IP) N(NATD_D_IP) N(FRAG_SUP) N(HASH_ALG) N(CHDLESS_SUP) N(MULT_AUTH) ] selected proposal: IKE:AES_CBC_256/HMAC_SHA1_96/PRF_HMAC_SHA1/MODP_1024 authentication of '172.20.253.237' (myself) with pre-shared key establishing CHILD_SA host-host{1} generating IKE_AUTH request 1 [ IDi N(INIT_CONTACT) IDr AUTH SA TSi TSr N(MOBIKE_SUP) N(NO_ADD_ADDR) N(MULT_AUTH) N(EAP_ONLY) N(MSG_ID_SYN_SUP) ] sending packet: from 172.20.253.237[4500] to 172.20.252.198[4500] (412 bytes) received packet: from 172.20.252.198[4500] to 172.20.253.237[4500] (220 bytes) parsed IKE_AUTH response 1 [ IDr AUTH SA TSi TSr N(MOBIKE_SUP) N(NO_ADD_ADDR) ] authentication of '172.20.252.198' with pre-shared key successful IKE_SA host-host[1] established between 172.20.253.237[172.20.253.237]...172.20.252.198[172.20.252.198] scheduling reauthentication in 9788s maximum IKE_SA lifetime 10328s selected proposal: ESP:AES_CBC_256/HMAC_SHA1_96/NO_EXT_SEQ CHILD_SA host-host{1} established with SPIs c4b20310_i c04d47dc_o and TS 172.20.253.237/32 === 172.20.252.198/32 peer supports MOBIKE connection 'host-host' established successfully administrator@ipsec-A:~$ sudo setkey -D 172.20.253.237 172.20.252.198 esp mode=tunnel spi=3226290140(0xc04d47dc) reqid=1(0x00000001) E: aes-cbc 7ed41201 e80a0bf4 fb871600 4cfcbfab 1e6e4fb3 c346376a 429d1575 1d58fc17 A: hmac-sha1 7a7a8e7d 7beaad8e 5f045ba1 45b42d7f bcc4de08 seq=0x00000000 replay=0 flags=0x00000000 state=mature created: Jun 14 20:37:14 2025 current: Jun 14 20:37:52 2025 diff: 38(s) hard: 3600(s) soft: 2765(s) last: Jun 14 20:37:14 2025 hard: 0(s) soft: 0(s) current: 3192(bytes) hard: 0(bytes) soft: 0(bytes) allocated: 38 hard: 0 soft: 0 sadb_seq=1 pid=28477 refcnt=0 172.20.252.198 172.20.253.237 esp mode=tunnel spi=3300000528(0xc4b20310) reqid=1(0x00000001) E: aes-cbc b834b5fd 2412b9cf 9e4ac726 29160d9e afbb91b0 7b2aafd5 07fc4c67 052b7958 A: hmac-sha1 31564ba0 bfdb79ff 872d5e0b fc3079d4 ea43a976 seq=0x00000000 replay=32 flags=0x00000000 state=mature created: Jun 14 20:37:14 2025 current: Jun 14 20:37:52 2025 diff: 38(s) hard: 3600(s) soft: 2819(s) last: Jun 14 20:37:14 2025 hard: 0(s) soft: 0(s) current: 3192(bytes) hard: 0(bytes) soft: 0(bytes) allocated: 38 hard: 0 soft: 0 sadb_seq=0 pid=28477 refcnt=0 administrator@ipsec-A:~$ sudo setkey -DP 172.20.253.237[any] 172.20.252.198[any] 255 out prio high + 1073374593 ipsec esp/tunnel/172.20.253.237-172.20.252.198/unique:1 created: Jun 14 20:37:14 2025 lastused: Jun 14 20:37:56 2025 lifetime: 0(s) validtime: 0(s) spid=265 seq=1 pid=28510 refcnt=1 172.20.252.198[any] 172.20.253.237[any] 255 fwd prio high + 1073374593 ipsec esp/tunnel/172.20.252.198-172.20.253.237/unique:1 created: Jun 14 20:37:14 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=258 seq=2 pid=28510 refcnt=1 172.20.252.198[any] 172.20.253.237[any] 255 in prio high + 1073374593 ipsec esp/tunnel/172.20.252.198-172.20.253.237/unique:1 created: Jun 14 20:37:14 2025 lastused: Jun 14 20:37:56 2025 lifetime: 0(s) validtime: 0(s) spid=248 seq=3 pid=28510 refcnt=1 (per-socket policy) in(socket) none created: Jun 14 20:34:15 2025 lastused: Jun 14 20:37:14 2025 lifetime: 0(s) validtime: 0(s) spid=243 seq=4 pid=28510 refcnt=1 (per-socket policy) out(socket) none created: Jun 14 20:34:15 2025 lastused: Jun 14 20:37:14 2025 lifetime: 0(s) validtime: 0(s) spid=236 seq=5 pid=28510 refcnt=1 (per-socket policy) in(socket) none created: Jun 14 20:34:15 2025 lastused: Jun 14 20:37:14 2025 lifetime: 0(s) validtime: 0(s) spid=227 seq=6 pid=28510 refcnt=1 (per-socket policy) out(socket) none created: Jun 14 20:34:15 2025 lastused: Jun 14 20:37:14 2025 lifetime: 0(s) validtime: 0(s) spid=220 seq=7 pid=28510 refcnt=1 (per-socket policy) in(socket) none created: Jun 14 20:34:15 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=211 seq=8 pid=28510 refcnt=1 (per-socket policy) out(socket) none created: Jun 14 20:34:15 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=204 seq=9 pid=28510 refcnt=1 (per-socket policy) in(socket) none created: Jun 14 20:34:15 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=195 seq=10 pid=28510 refcnt=1 (per-socket policy) out(socket) none created: Jun 14 20:34:15 2025 lastused: lifetime: 0(s) validtime: 0(s) spid=188 seq=0 pid=28510 refcnt=1 administrator@ipsec-A:~$
Questions
How many SA associations have been negotiated?
The setkey -D result shows two ESP associations in the SAD database (SPI 0xc04d47dc and 0xc4b20310) - one for each direction of movement.
What type of IPSec header and mode was used for the SA associations created? What algorithms were used?
According to the ipsec log up host-host, ESP was negotiated in tunnel mode (ESP:AES_CBC_256/HMAC_SHA1_96/NO_EXT_SEQ).
Algorithms: AES-CBC 256-bit for encryption and HMAC-SHA1-96 (96-bit tag) for authentication.
What is the key length used for each algorithm?
AES-CBC-256 uses a key length of 256 bits.
HMAC-SHA1-96 uses a tag of 96 bits (12 bytes), although the HMAC key itself can be longer (typically 160 bits), in the log we see 20 bytes of key, but the tag is truncated to 12 bytes.
In which phase of the IKE protocol was the shared PSK key used?
The PSK key was used in phase 1 (IKE_SA_INIT and IKE_AUTH) to authenticate the endpoints.
What SPD policies were created by the IKE process?
setkey -DP shows four entries:
two out and in entries for ESP tunnel mode (one each for A→B and B→A traffic) with reqid = 1 and unique flag.
an additional fwd entry related to packet forwarding.
Policies apply to traffic 172.20.253.237[any]. ↔ 172.20.252.198[any] and have priority 'high', operation ipsec esp/tunnel/…/unique:1.
Analysis of IKE traffic in Wireshark
Filter: udp.port == 500 or udp.port == 4500.
The IKE_SA_INIT packets show the exchange of SA offers (AES_CBC_256/HMAC_SHA1_96/PRF_HMAC_SHA1/MODP_1024) and DH keys.
In IKE_AUTH, IDi, IDr and AUTH payload with PSK authentication appear.
